Without warning Sue and old Alf explode (3,2,1,6)
I believe the answer is:
all of a sudden
'without warning' is the definition.
(I've seen this before)
'sue and old alf explode' is the wordplay.
'explode' is an anagram indicator.
'sue'+'and'+'old'+'alf'='sueandoldalf'
'sueandoldalf' anagrammed gives 'ALL OF A SUDDEN'.
(Other definitions for all of a sudden that I've seen before include "abruptly" , "Without warning" , "Quickly and unexpectedly" , "In an unexpected manner" .)